Infiltration is the diffusion or accumulation (in a tissue or cells) of foreign substances in amounts excess of the normal. The material collected in those tissues or cells is called infiltrate.

As part of a disease process, infiltration is sometimes used to refer to the invasion of cancer cells into the underlying matrix or the blood vessels. Similarly, the term may describe the deposition of amyloid protein. During leukocyte extravasation, white blood cells move in response to cytokines from the blood into diseased or infected tissues, usually in the direction of a chemical gradient,^[1] in a process called chemotaxis. The presence of lymphocytes in tissue in greater than normal numbers is likewise called infiltration.

As part of medical intervention, local anaesthetics may be injected at more than one point so as to infiltrate an area prior to a surgical procedure.

The term may also be used to refer to extravasation.

• Dorland's Illustrated Medical Dictionary (Twenty-sixth ed.). ISBN 0-7216-1645-3.
• Vinay Kumar, Abul Abbas, and Jon Aster. Robbins and Cotran Pathologic Basis of Disease (9th ed.).{{cite book}}: CS1 maint: multiple names: authors list (link)